The purpose of these reactions is to release energy stored in the sugar molecule. To explain that process, one must know that a sugar molecule consists of carbon, hydrogen, and oxygen atoms held together by means of chemical bonds. A chemical bond is a force of attraction between two atoms. That force of attraction is a form of energy. A sugar molecule with two dozen chemical bonds can be thought of as containing two dozen tiny units of energy. Each time a chemical bond is broken, one unit of energy is set free.
Phenylketonuria (PKU) is caused by decreased activity of phenylalanine hydroxylase (PAH), an enzyme that converts the amino acid phenylalanine to tyrosine , a precursor of several important hormones and skin, hair, and eye pigments. Decreased PAH activity results in accumulation of phenylalanine and a decreased amount of tyrosine and other metabolites. Persistent high levels of phenylalanine in the blood in turn result in progressive developmental delay, a small head circumference, behaviour disturbances, and seizures. Due to a decreased amount of the pigment melanin , persons with PKU tend to have lighter features, such as blond hair and blue eyes, than other family members who do not have the disease. Treatment with special formulas and with foods low in phenylalanine and protein can reduce phenylalanine levels to normal and maintain normal intelligence. However, rare cases of PKU that result from impaired metabolism of biopterin, an essential cofactor in the phenylalanine hydroxylase reaction, may not consistently respond to therapy.
Metabolism is a generic term for all the chemical reactions that break down or "burn" food to provide energy for the operation of an organism. The word "burn" is used advisedly, because the energy yield from a food in the human metabolic process is comparable to the energy obtained by actual combustion. The energy available from a food is commonly stated in dietary Calories, and the Calorie rating of a food may actually be obtained by burning it in a pure oxygen atmosphere in a calorimeter to measure the energy yield from this combustion.